This invention relates to regulated adjustable electrical power supplies and, in a more specific context, to a regulated adjustable power supply for a microwave magnetron of a microwave oven.
The microwave oven is the familiar appliance used to heat or cook foods by exposure of same to microwave energy radiation. For this purpose conventional microwave ovens employ an electonic vacuum tube, known in the art as a magnetron. Simply stated, the magnetron is a device having unidirectional current carrying characteristics which converts DC voltage and current into energy of the microwave frequency range such as the frequency permitted by law, 2,450 megahertz. To provide that DC voltage, various additonal electrical components are included as a power supply to convert normal household line voltage, typically 120 or 240 volts, 50 to 60 hertz, to the high voltages, on the order of 3,000 to 4,000 volts DC, required in the operation of presently available magnetrons. In its essentials, present microwave oven power supply contains a transformer for stepping up the 120 volt AC to the level of 3,000 to 4,000 volts, depending upon the particular voltage requirement of any specific model of microwave magnetron, a rectifying means, or a voltage doubler-rectifier, and the magnetron itself. And a source of low voltage is provided for the heater of the magnetron. Microwave energy generated by the magnetron is taken from the magnetron output and transmitted either directly into the oven chamber or through a waveguide to the oven chamber.
The average power supplied to the magnetron is set within limits by the design of both the power supply and the magnetron and is generally directly related to the microwave output power generated thereby. It is known that the adjustment of the microwave power can be made within limits by adjustment of the DC current level through the magnetron. Present microwave oven power supplies generally employ a high leakage reactance transformer in combination with a modified half-wave voltage doubler, known also as the "Villard" circuit, to rectify and double the voltage output of the high voltage transformer, and apply a high voltage DC to the magnetron. Examples of such circuits appear in the following patents: U.S. Pat. No. 3,396,342 to Feinberg: U.S. Pat. No. 3,651,371 to Tingley; and U.S. Pat. No. 3,684,978 to Otaguro, which have been made known to petitioners. The circuit provides satisfactory operation and uses a minimum number of components. Recent practice is to provide additional elements within the oven power supply which permit the user to adjust the average power of the magnetron. This has been accomplished as either a two-step "high" or "low power" arrangement or as an adjustable level device allowing continuous adjustment. In the first type of device the value of the capacitance was changed in order to vary the current, such as illustrated in U.S. Pat. No. 3,684,978 to Otaguro. Still another is to employ a Triac control in the primary circuit of the transformer in order to regulate the average amount of current into the power supply, such as found in the Litton Microwave Ovens sold under the brand name "Varicook". In the Varicook circuit a separate filament transformer is required because of the interrupted voltage into the primary of the high voltage transformer, and the expedient of having the filament winding combined as a separate winding upon the high voltage transformer cannot be employed. Additonally the use of pulse techniques, inherent in the Varicook method, in the primary winding of the transformer creates additional stresses on the transformer insulation, which are best avoided.
In the case of current control in the secondary winding circuit, the approach set forth in U.S. Pat. No. 3,684,978 patent does not provide sufficient variety of adjustment, and an adjustable resistor approach disclosed in U.S. Pat. No. 3,760,291 made known to us, appears somewhat impractical and wasteful in that a resistor consumes electrical energy and generates heat.
The present invention relates to the control of the average power output level of a magnetron by controlling the voltage in the secondary of the transformer. More particularly, the invention provides for a control which can be used to allow the user to selectively adjust the power level of the magnetron within a certain range, or, in an alternative application, which may be established adjusted to a fixed level at the factory. In so doing, the filament voltage may be supplied by an additional winding on the same high voltage transformer as is used to provide the high voltages to the magnetron. The primary pulse techniques of the Varicook technique is avoided, improving the reliability of the transformer, and any surges caused by the lightning on the input line as might destroy semiconductor type control devices connected in the primary circuit are minimized.